Natural gas is a naturally occurring hydrocarbon gas mixture. Natural gas includes mainly saturated hydrocarbon components such as methane, ethane, propane, butane, and heavier hydrocarbons. Natural gas typically contains about 60-100 mole percent methane based on the total hydrocarbon content, with the balance of the hydrocarbon content being primarily heavier alkanes. Alkanes of increasing carbon number are normally present in decreasing amounts. The components of natural gas have many uses. For example, these can be used as a source of energy for heating, cooking, electricity, and pressure generation. The components also may be used as chemical feedstock in the manufacture of other chemicals, as fertilizers, as animal and fish feed, and the like. However, the components often are separated in order to be more suitable for a desired use.
“Raw natural gas” refers herein to natural gas as obtained from natural sources. In addition to hydrocarbons, raw natural gas may include other constituents including one or more of carbon dioxide, water, nitrogen, hydrogen sulfide, mercaptans, mercury, chlorides, helium, or the like. In some applications, these additional constituents are undesirable contamination and are removed in order to convert the natural gas into one or more useable products. In many desirable modes of practice, raw natural gas is treated using one or more purification processes in order to remove one or more of such contaminants to a desired degree. As used herein, the term “natural gas” will refer to raw natural gas that comprises at least one of C1 and/or C2 hydrocarbons as well as one or more C3+ hydrocarbons and that has been treated to remove at least a portion of one or more contaminants.
It is often desirable to separate natural gas into one or more light components (e.g., C1 and/or C2 enriched components) or heavy components (e.g., enriched in one or more C3+ hydrocarbons referred to herein as “natural gas liquid” materials). For example, it is financially desirable to recover natural gas liquids from natural gas to be used as petrochemical feedstocks where they have a higher value as compared to their value as a fuel gas component. Another reason is to meet pipeline specifications or liquefied natural gas (LNG) specifications for heating value, dew point, and condensation.
Moreover, oilfields are often located in remote locations where power grids have not yet been developed and electrical power is not available. For example, fuels such as diesel may be needed to run onsite oilfield equipment at remote locations. While natural gas is often readily available in such remote locations, the use of raw gas is not feasible unless a sufficient amount of the natural gas liquids have first been removed. Otherwise, natural gas containing too much NGL content may have elevated BTU levels and may not be suitable for gas combustion systems that are designed to operate within a narrow BTU range. Using a natural gas with too high of BTU level may require higher maintenance costs, higher operating temperatures, reduced equipment life expectancy, decreased power reduction, and/or generate increased pollution if operated at higher BTUs.
Many techniques for separating natural gas into desired components are known. Techniques include adsorption, gas-liquid separation, and combinations of these. Pressure swing adsorption (PSA) is one exemplary separation technique. In a conventional PSA process, an adsorbent is used that selectively adsorbs higher molecular weight hydrocarbons relative to methane and ethane under an elevated pressure, but then will readily release the adsorbed material when the pressure is reduced. This allows lighter components to be recovered in a first stage while heavier components are adsorbed under pressure. In a second stage, the heavier components can be separately recovered by releasing the pressure, which also regenerates the adsorbent for further use.
The light hydrocarbon stream resulting from adsorption may be highly purified with respect to C1 and/or C2 hydrocarbon content while containing very little C3+ hydrocarbon content. In the meantime, the recovered heavy material may be enriched with respect to C3+ hydrocarbon content but may still include more C1 and/or C2 content than may be desired. Accordingly, the resulting heavy hydrocarbon stream is further purified to remove more of the C1 and/or C2 content. This may be accomplished using liquefaction (gas-liquid) separation strategies in which some of the NGLs are condensed to separate them from lighter components in a gas phase.
Thus, a goal of natural gas separation is to obtain both a highly purified natural gas product containing predominantly C1 and/or C2 hydrocarbon material and a purified NGL product containing predominantly C3+ hydrocarbon material. However, it has been difficult to use liquefaction strategies to remove both C1 and C2 hydrocarbon material from the heavy stream. The result is that the heavy stream may include too much C2 content such that the C3+ hydrocarbon content remains more dilute than desired. Improved strategies to recover more concentrated heavy streams that are more easily resolved from both C1 and C2 hydrocarbon materials are desired.